Rob Portman, a leading Republican senator, has come out in favour of same-sex
marriage after learning that his own 21-year-old son is gay.

Mr Portman said he was abandoning his opposition to gay marriage because because he wanted his son Will to have "the joy and the stability" that legally-recognised marriage gives to heterosexual couples.

The senator said he reacted with "love" and that he had been honest about his son's sexuality while being vetted as a potential running mate for Mitt Romney last year. He added that he did not believe the issue cost him the vice-presidential slot.

"I’ve thought a great deal about this issue, and like millions of Americans in recent years, I’ve changed my mind on the question of marriage for same-sex couples," he wrote. "As we strive as a nation to form a more perfect union, I believe all of our sons and daughters ought to have the same opportunity to experience the joy and stability of marriage."

Mr Portman's Ohio is one of 37 states that explicitly bans same-sex marriage but the senator said he would not become an activist on the issue and would instead focus on economic policy.

Nine states plus Washington DC recognise gay marriage and polls show a growing majority of Americans support legalising gay marriage.

However, the overwhelming majority of Republican politicians remain opposed. Mr Portman said he had spoken to Dick Cheney, the former vice president who became a supporter after his own daughter came out as a lesbian. Mr Cheney told him to "follow your heart".

Later this month the US Supreme Court will hear two cases on the issue of gay marriage. One will consider the constitutionality of the 1996 Defence of Marriage Act (DOMA), which forbids the federal government from recognising same-sex marriages.

The other is focused on California's Proposition 8, which explicitly bans gay marriage in the state. Both cases have the potential to be landmark rulings but the Proposition 8 case is particularly significant and could lead to the Supreme Court declaring that gay people have a constitutional right to marry.

If they reach that conclusion it could nullify all existing legislation against gay marriage.

Mr Portman said that he, like the White House, believed that DOMA should be overturned and decided to make his position clear ahead of the the case being heard on March 28.